The present invention relates to a process for producing an .alpha.-aromatic group substituted alkanoic acid derivative represented by formula IV (hereinafter referred to as compound IV). ##STR3## wherein Ar represents a substituted or unsubstituted aryl group having 6 to 12 carbon atoms or a heterocyclic group, R.sup.3 and R.sup.4 are the same or different groups and each represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms and R.sup.5 represents a hydrogen atom or a substituted or unsubstituted alkyl group having 1 to 5 carbon atoms.
Heretofore, various processes for producing compound IV have been known. Among these processes is a process for producing compound IV from a compound represented by formula I (hereinafter referred to as compound I). ##STR4## wherein R.sup.1 and R.sup.2 are the same or different groups and represent an alkyl group having 1 l to 5 carbon atoms or form a cyclic acetal of a single compound and Ar, R.sup.3 and R.sup.4 have the same significance as described above, which process comprises tosylating the hydroxy group and heating compound I in, for example, an aqueous calcium carbonate solution to obtain compound IV [Tsuchihashi et al, Tetrahedron Letters, 22, No. 43, 4305 (1981)].